The polymeric monolithic capacitor (PMC) has a polymer dielectric and metallized electrodes and is a structure generally similar to that of a metallized film capacitor (MFC), with the notable exception that metallized film capacitor is produced by winding or stacking a metallized film. As a result of the corresponding process of manufacture, an MFC does not possess a monolithic structure, because it is produced in atmospheric conditions and includes air-filled micro-gaps between the constituent layers. Furthermore, metallized films have a minimum thickness, which is determined by the polymer chemistry, the process of manufacture and the fact such films must be strong enough to be handled during the manufacturing process, metallization, slitting into bobbins and winding into a capacitor.
In stark contradistinction to the MFC, the PMC capacitor is produced in the vacuum by forming thousands of polymer dielectric layers and metallized electrode layers in a single process step. The lack of air between the layers and interlayer bonding turns the resulting structure into a substantially monolithic structure. The individual polymer dielectric layers of this structure are pinhole free, and the overall structure is not touched or brought in contact with anything or exposed to air during the process of manufacture until and after a stack of thousands of polymer/metal layers has been already formed (including protective layers on the bottom and top surface of the stack). As a result, the polymer dielectric layers of the PMC structure can be formed to be as much as 100 times thinner that common polymer film dielectrics. This advantage, combined with a wide range of physical and dielectric properties of the PMC structure, results in volumetrically efficient capacitors that can be used in applications that are traditionally served by other capacitor technologies (such as, for example, MFCs, electrolytic and Ceramic Multilayer Capacitors (MLCs).
Applications that involve operation at high temperatures, high ripple current, high voltage and current pulses, and high energy density, are currently served by capacitors fabricated with various technologies including MFCs, aluminum and tantalum electrolytic capacitors, and MLCs. For example, MFCs are used extensively in a broad range of electrical and electronic equipment. In order to reduce the dimensions of an MFC—an ever-popular task—the thickness of the polymer film layer(s) of such capacitor should be reduced. The degree to which the reduction of the thickness of the polymer film (such as, for example, a polypropylene (PP) film) can be achieved is limited by the film-manufacturing process, and the resulting thickness of PP films is typically no less than about two microns. This limits the voltage at which such film capacitor can be used to about 300 VDC. Therefore for a low voltage application—for example the one requiring 25 V, that requires a capacitor with the high quality properties of PP film, the use of a PP MFC is prohibitive due to its large size and high cost. Such low voltage applications will typically be serviced with aluminum electrolytic capacitors when the applications require high capacitance, and MLCs when the applications require lower capacitance values. PMC capacitors with submicron polymer dielectrics are small and can be used to replace both aluminum electrolytics and MLCs, with the added benefit of the stable polymer dielectric properties.
Applications such as those employing DC-links (for example, used in inverters for hybrid and electric vehicles) utilize metallized PP film capacitors to minimize ripple current, voltage fluctuations, and to suppress transient effects. Key characteristics of such DC-link capacitors used in voltage-sourced inverters of electric drive vehicles include self-healing properties to assure a benign failure mode, withstanding high ripple currents, low dissipation factor (DF), high capacitance, and high operating temperature. The list of these requirements effectively excludes the use of electrolytic capacitors and MLCs. Metallized PP capacitors that are almost exclusively used in such DC-link applications, have an operating temperature limited to 105° C. with significant derating in voltage, ripple current and capacitor lifetime, and such capacitors are relatively large and costly. Therefore, there remains a need—at least in the automotive industry—to reduce the DC-link capacitor size and to extend the capacitor's upper operating temperature to at least 125° C. and preferably as high as 140° C.
Yet another group of applications in which the capacitors play a critical part in the final product is pulse power systems that deliver a pulse of energy in a short time period. Here, the search for smaller and better-performing capacitors remains ongoing. Examples of such applications include implantable defibrillators and a multitude of defense applications where size and weight are critical capacitor parameters. For example, in an implantable defibrillator, the capacitor occupies about 50% of the defibrillator volume and the aluminum and tantalum electrolytic capacitors that are currently used in this application can weigh more than all the other defibrillator components combined. At another extreme, a metallized PP capacitor bank used to fire a rail gun that could potentially be used in a tank, is currently larger than the tank itself. Therefore, there in a need to develop new capacitor technologies to maximize operating temperature, to handle higher ripple and pulse currents, and to reduce the capacitor weight and volume, while improving capacitor lifetime and reliability.